Margin of Error: Exact
Corners, both inside and outside, are the more demanding part of hanging paper. In and of themselves, they are not that difficult. The problem is that the corners are seldom true plumb or vertical, and this is where the difficulty lies. You need to be able to hang your wallpaper plumb, even if the corners are not. This can be demanding and can take some time and care. Go slowly here, since this is where your new skills will be challenged the most.
At both the top and bottom of the wall, measure the distances from the strip next to the corner to the actual corner itself. Add 1" to the greater of the two measurements and cut a sheet lengthwise to this measurement If, however, this measurement is within 6" of your full sheet measurement, use a full sheet. If there is a sheet next to the corner, its edge can be used as a plumb line (let's hope that you have installed it plumb).
If there is no sheet at the corner, simply measure out from the corner the dimensions of a wallpaper roll less 1" (for corner overlap) and draw a plumb line. This line will then be your guideline in hanging your first sheet. Since many times your first sheet is installed in an inconspicuous corner, this is the process you will use to install your first sheet
Now simply hang the sheet as described earlier and wrap the excess into the corner. Since few corners are perfectly plumb, you will need to strike a plumb line on the adjacent wall, again at a distance from the corner of 1" less than the dimension of the roll. Then apply another sheet and wrap the excess so it overlaps the first sheet. Now simply take your broad knife and your razor knife and cut the overlapping sheets in the corner and peel away the two excess pieces.
If the outside corner is exactly plumb, and they seldom are, you can simply wrap the paper around the corner and begin from its edge on the other side of the corner. If it is not plumb, a little more attention is needed.
As with inside corners, measure at the top and bottom of the wall the distance from the last sheet to the corner. Add 1" to the longest measurement and cut a sheet that size. If the measurement is within 6" of a full sheet, use a full sheet Hang this sheet, cutting a diagonal slit at the corner at the top so it will bend around the corner.
Hang it so that it is smooth and fold it smoothly around the corner. Now you need another plumb line on the intersecting wall. To do this, strike a plumb line on this wall with your wallpaper level. The plumb line should be the width of the roll from the corner. Now simply hang your intersecting piece to that plumb line. After both pieces are in place, make a new plumb line 1/4" 1/2" from the corner on the side of the corner. At this line, cut through both pieces of paper and peel away the excess.